I think the `57 body may have a few more bolts under the back seat that the `66 pan doesn't use so those holes'll need to be covered. The jack ports of a sedan pan have to be amputated to clear the reinforcement rails that run along the outside of the convertible body: http://www2.cip1.com/photos/VWC-151-801-131-B-2T.jpg If you're using the stock `57 28PCI carb with manual choke you'll need to figure out how to route a cable to it. The pedal cluster design was changed in mid`66 to the late style with the accelerator pedal hinged to it rather than the floor; the floorboards have a slightly different contour under the pedals and the pedal stop is different, also the lateral position of the clutch tube inside the tunnel was changed in `64 necessitating a "hook" with a different offset on the end...so if you try to use the `57 pedal cluster in the `66 the least you'll have to do is change the hook.
Other than that it should be pretty straightforward. Be sure to check with your DMV/law enforcement agencies to see what paperwork/inspection may be needed to make it legal before you dive in.